Yeast - A Treatise - Section I

12-25-2001

Table of Contents

Preface

A baker learns early in the baking process that it is difficult to make fine bread unless he or she gains a fair degree of insight into many of the chemical, physical, biological and mechanical aspects of the baking craft.  It is fitting that one of the seemingly most simple of organisms - a yeast cell - offers challenges that defy that assumed simplicity.  Yeast is a very complex organism, and its effects on baking are complex.  In this treatise we have tried to review as much of the literature available to us, and to distill it into a reasonably brief review of that literature. Since The Artisan has no research facilities capable of doing independent research aimed at better understanding what yeast does and why it does it,  we have relied on many sources.  However, that does not mean that any errors or misconstrued conclusions are the fault of those sources.  Errors of either commission or omission are ours, and ours alone.  We hope that visitors will inform us of any errors that we have made, and allow us the opportunity to correct said errors as appropriate.

Source materials for this Treatise have come from those authors and the works cited in the Bibliography found at the end of this document.

Introduction
The baking process represents a highly complex set of physical, chemical, biochemical and biological activities. The microscopic yeast cell is responsible for the most important of these - Fermentation. Thus, yeast is the primary biological agent in dough formation, and discussions of yeast and its functions in the baking process are invariably intertwined with those pertaining to fermentation, and visa versa.

What are yeasts? Yeasts belong to the phylum Thalophytes. Members of this phylum form the most basic division of organisms in the plant kingdom, and are an undifferentiated group. Yeasts belong to this phylum along with other funghi, algae, and bacteria. Since funghi lack chlorophyll, they are dependent for food upon other organism's production of organic food matter. (Pyler) Thus, yeast must be fed to accomplish the task of leavening the dough. Yeast used in bread baking belongs to the genus Saccharomycetes and the species cerevisiae. More about this below.

Biology of Yeast Cells - Simplified
Yeast are a tiny form of fungi or plant-like microorganism (visible only under a microscope) that exist in or on all living matter i.e. water, soil, plants, air, etc. A common example of a yeast is the bloom we can observe on grapes. As a living organism yeast needs sugars, water and warmth to stay alive. In addition, albumen or nitrogenous material are also necessary for yeast to thrive.

There are hundreds of different species of yeast identified in nature, but the genus and species most commonly used for baking is Saccharomyces cereviae. The scientific name Saccharomyces cerevisiae, means 'a mold which ferments the sugar in cereal (saccharo-mucus cerevisiae) to produce alcohol and carbon dioxide'. Yeast needs energy to survive, and has a number of ways to attain that energy.   Fermentation and respiration are two ways   The ultimate reaction of importance in this process is the an-aerobic conversion of simple sugars to ethyl alcohol and carbon dioxide during alcoholic fermentation as shown below. Although not shown in the fermentation reaction,  numerous other end products are formed during the course of fermentation

Simple Sugar      →      Ethyl Alcohol + Carbon Dioxide
C6 H12 O6     →     2C H3 CH2 OH + 2CO2

The basic respiration reaction is shown below.   The differences  between an-aerobic fermentation and aerobic respiration can be seen in the end products. Under aerobic conditions, yeasts convert sugars to carbon dioxide,  water and cellular mass. (Giorilli & Lauri)

Simple Sugar + Oxygen       →    Carbon Dioxide + Water
C6 H12 O6 +  6O2   →      6CO2 + 6H2O

Examining a yeast cell under a microscope will give a greater understanding of the composition and nature of yeast. The method for viewing a sample of yeast under a microscope is to disperse a small amount of yeast in water, causing the water to be slightly clouded, and then drop a spot of the liquid onto a glass slide. The drop is then covered and viewed with a 650 x magnification. The individual cells will take the general form illustrated in Figure 1.

When viewed under the microscope, one sees round or oval cells about 1/100 of a millimeter in diameter, which weigh about 8 to 10 billion to the gram. (Calvel et al) If individual yeast cells were placed side by side it would take approximately 1200 cells to measure 1cm in length. Inside each cell are the following:

  • A liquid solution of protoplasm, protein, fat and mineral matter.

  • One or more dark patches called vacuoles.

  • A darker spot which is the nucleus. This is where the cell's genetic information is stored as  DNA   which controls all the operations of the cell.

A yeast cell has 6000 different yeast genes. Like any living thing, yeast is made up of chromosomes; there are 16 different chromosomes in yeast compared with 23 in humans.

The double cell wall may have bud scars (seen in Figure 1 to the right), which are caused by budding, i.e. the cell reproducing itself. There can be up to ten such scars, which cover the cell totally, after which the cell expires.

This happens (generally speaking) as follows:

Compared with other plant organisms, yeast has a much better chance of survival in spite of harsh environmental conditions. It is independent from climate and soil conditions. It is not dependent on any location and can survive for hundreds of years as a spore.

Under favorable living conditions, yeast multiplies through the separation of cells (budding) or yeast multiplication. Under unfavorable living conditions, when water and nutrients are lacking, the yeast forms spores.

Cell Separation (Budding)
The cell core migrates to the cell wall of the yeast cell. It splits up and forms a daughter cell. The daughter cell multiplies in the same way while it is still growing and tied to the mother cell. A colony develops. Later, the daughter cell separates from the mother cell. The multiplication process continues for as long as the conditions for multiplication are present. This is depicted in Figure 2. As can be seen, a parent cell grows a protuberance, this swells as the bud forms, a neck develops between the parent cell and the bud, and they separate. The process starts again and, in ideal conditions, a cell can reproduce itself in 20 minutes so that numbers increase from one to two, then to four, to eight, to 16, and so on. If the numbers are plotted on a graph, the line would take an exponential form.

Figure 2


Sporulation
Spores form once the nutrients of a solution are used up. The yeast becomes dormant and feeds on its reserve material. When the nutrient solution and the yeast cells dry out, the cell core separates and forms spores. The spores are insensitive to heat and cold. The slightest breeze carries them anywhere. Under dry conditions, the spores can live forever. When spores fall into a nutrient solution, they germinate into yeast cells. Each yeast cell can give rise to four spores.

Yeast Production - General Discussion

Many scientific publications emphasize varied aspects of the commercial production of bakers yeast. Companies producing bakers yeast offer different guidelines relative to the appropriate application of their products. What follows is an attempt to synthesize and clarify some of the available information.

An enormous number of strains of Saccharomyces cereviae exist, many of which have already been selected for baking. Among the characteristics individual strains share are the substances they use for growth, especially how and which sugars they utilize, as well as the manner in which they reproduce, and their appearance through the lens of a microscope. Characteristics differentiating individual strains include how much sugar they tolerate, and how quickly they grow. The potential characteristics of a particular baker's yeast are determined by the strain of yeast that is selected. The actual characteristics of bakers yeast from a particular strain are determined by its composition.

The following is a simplified description of the production of bakers yeast: Yeast needs moisture, food, and warmth to grow. As a living organism, yeast is cultivated, rather than manufactured. A carbohydrate food source such as molasses is required for reproductive yeast growth. Molasses is a by-product of sugar beet and sugar cane refining, and supplies the least expensive source of sucrose, glucose and fructose, in addition to some minerals and assimilable nitrogen. Either beet or cane molasses, or a combination of both, can be used. Prior to its use, the molasses is diluted with water, clarified, and heat sterilized. The molasses is supplemented with nitrogen, phosphate, sulphuric acid and sodium carbonate, and small amounts of minerals and trace elements. Nitrogen is supplemented using ammonia or various ammonium salts. Phosphate is supplied in the form of phosphoric acid or ammonium or di-ammonium phosphate. Sulphuric acid and sodium carbonate are included as processing aids for pH control. Minerals such as calcium and magnesium are added as are trace amounts of iron and zinc. Oxygen is provided in the form of filtered air. All of these substances are contained in an aqueous (water) solution or broth referred to as the "wort."

Appropriate strains of yeast cultures are multiplied in phases. Starter cultures are grown under strictly sterile conditions in a laboratory. Single healthy and vigorous cells are selected from desired yeast strains. They are isolated for growth on pure culture slants, or in test tubes containing the required nutrients. This enables them to begin the process of reproduction. These pure cultures are subsequently transferred to flasks, then larger vessels until they have grown to the extent required for a commercial starter.

Yeast propagation proceeds in a series of increasingly larger sanitized tank fermenters. This large-scale fermentation continues under controlled conditions of continuous nutrient addition, temperature control, and optimum aeration. Conditions of growth affect how fast yeast multiplies and how much protein and carbohydrate it accumulates. For instance, rapid growth usually results in yeast with higher protein and enzymes, lower carbohydrates, higher initial activity and lower stability. Alternatively, slow growth usually results in yeast with lower protein and enzymes, higher carbohydrates, lower initial activity, and higher stability.

When all of the nutrients have been consumed, the yeast cells are separated from the remaining nutrient matter by centrifugation (separator). They are then washed and recentrifuged to yield a creamy suspension of pure, active yeast known as "yeast cream" which has a solids (yeast cells) content of approximately 15-18%. From this stage, a series of manufacturing processes take place that prepares the yeast for its final form. These are described briefly below:

Compressed Yeast (also called cake, wet, and fresh yeast)

The water content of the yeast cream is further reduced by passing the yeast cream through a filter press or rotary vacuum filtration unit. Once pressed, the pressed cake is extruded through a rectangular nozzle to form a strand that is cut into the proper length and weight. The strain chosen for compressed yeast and the growth conditions tend to favor high activity or gassing power in both lean and sweet dough.

Active Dry Yeast

Active dry yeast begins as compressed yeast but the press cake is extruded through perforated plates or screens in the form of thin spaghetti-like strands. These strands are cut into elongated pellets as they enter a tunnel dryer and pass through a series of drying chambers maintained at different temperature levels. (Drying may also be carried out in rotating drums or in the fluid bed system.) The pellets are then ground into small granules or beads. The combination of strain chosen for active dry yeast, the growth conditions, and the drying method tend to favor stability over activity. This means that active dry yeast has lower activity or gassing power than compressed yeast in lean dough. Active dry yeast has lower activity or gassing power than compressed yeast and higher activity or gassing power than instant active dry yeast in sweet dough.

Instant Active Dry Yeast

The manufacturing process for instant active dry yeast is similar to that for active dry yeast with a few exceptions. Ascorbic acid may be added as a dough conditioner to help strengthen the dough. Prior to being extruded, the press cake may be plasticized with sorbitan monostearate (an emulsifier) as an aid to yeast rehydration in the dough. The yeast mass is extruded through smaller perforated plates or screens than those used for active dry yeast, cut into small oblong, thread like particles and dried in a fluid bed dryer. The combination of strain chosen for instant yeast, the growth conditions, the drying method, and the addition of emulsifiers tend to place instant yeast intermediate between compressed and active dry yeast relative to activity or gassing power in lean dough. Instant active dry yeast has lower gassing power than compressed or active dry yeast in sweet dough.

Yeasts Available to Bakers
The following information is typical for each type of yeast, but may vary somewhat according to product and company:

Compressed Yeast (also called cake, wet, and fresh yeast)

Fleischmann's compressed yeast is available in supermarkets in 0.6 oz cakes, and Red Star compressed yeast is available in some supermarkets in 2 oz. cakes. It is found in the dairy or deli case. Compressed yeast is available to commercial bakers from a variety of companies in 1 and 2 pound packets. Compressed yeast has approximately 30% solids and 70% moisture content. It is highly perishable and must be stored at a uniformly low temperature (about 40 F) to prevent excessive loss of activity or gassing. Compressed yeast generally has a shelf life of approximately two weeks from its make or packaging date when kept at 73.3 degrees F. (23 degrees C)

At 32-42 F. (0 - 5.5 C) compressed yeast loses approximately 10% of its gassing power over a 4 week period.  At 45 F (7.2 C) yeast will lose 3-4% of its activity per week.  At 95 F (35 C), one half of the gassing power is lost in 3-4 days.  Once yeast starts to deteriorate or lose its fermentative activity, it does so quickly, losing almost all of its activity (autolysis) by the third week.  It has, however, been shown that compressed yeast can be successfully stored  for two months at 30 degrees F. (-1 degree C).  When this is done, good bread can be made from yeast stored for two, but not three,  months. 

To use compressed yeast, crumble it into the dry ingredients or soften it in tepid water.


Active Dry Yeast

Fleischmann, Red Star, and SAF active dry yeast are available in supermarkets in 1/4 oz (7 g) packets and/or 4 oz (113.4 g) jars. Active dry yeast is available to commercial bakers from a variety of companies in 1 and 2 pound, and 500 g packets. It also is available in these sizes to home bakers at warehouse or club stores, and via mail order. Active dry yeast has approximately 92.0% solids and 8.0% moisture content. It is advisable to store active dry yeast in a cool, dry place that does not  exceed 80F. The shelf life of active dry yeast stored at room temperature is approximately 2 years from its make date. Once opened, active dry yeast is best stored in an airtight container in the back of the refrigerator, where it will retain its activity for approximately 4 months. To rehydrate active dry yeast, blend one part yeast with four parts lukewarm water, wait 10 minutes, and stir. Depending upon the particular product and company, lukewarm water ranges from 90-115 F. Temperatures lower than 90 F and higher than 115 F should be strictly avoided. (Pyler) Active dry yeast may also be blended with the dry ingredients.

Instant Active Dry Yeast

Fleischmann, Red Star, and SAF instant active dry yeast is available in supermarkets in 1/4 oz (7 g) packets and/or 4 oz (113.4 g) jars. The Fleischmann product is marketed as RapidRise, the Red Star product is marketed as QUICK.RISE, and the SAF product is marketed as Gourmet Perfect Rise. Fleischmann also markets an instant active dry yeast named Bread Machine Yeast. Instant active dry yeast is available to commercial bakers in 1 and 2 pound, and 500 g packets. It also is available in these sizes to home bakers at warehouse or club stores, and via mail order. Instant active dry yeast has 96.0% solids and 4.0% moisture content. It is advisable to store instant active dry yeast in a cool, dry place that does not exceed 80 F. The shelf life of instant yeast stored at room temperature is approximately 2 years from its make date. Once opened, instant active dry yeast can be stored in an airtight container in the back of the refrigerator, where it will retain its activity for approximately 4 months. Three methods are recommended when using instant active dry yeast: The first is to blend it thoroughly with the flour before adding water. The second is to mix all the ingredients except the instant yeast for one to two minutes, sprinkle the instant yeast on top of the partially mixed ingredients, and continue mixing. The third is to blend one part yeast with five parts lukewarm water, wait 10 minutes, and stir.

Osmotolerant Instant Active Dry Yeast
A yeast product available commercially but not readily available to home bakers is osmotolerant instant active dry yeast. Osmosis is the means by which yeast cells absorb oxygen and nutrients and give off enzymes and other substances. Osmotolerant instant active dry yeast is recommended for use in dough characterized as sweet, salty or low absorption. A yeast with good osmotic tolerance is called for because the amount of available water in these doughs is limited. Fermipan markets osmotolerant instant active dry yeast as Fermipan Brown and SAF markets osmotolerant instant active dry yeast as SAF Gold. In the absence of osmotolerant instant active dry yeast, it is sometimes recommended that the amount of active dry or instant active dry yeast in a sweet, as opposed to a lean, dough be increased.

It is worth noting that there is disagreement among the yeast companies as to whether or not active dry and instant active dry yeast should be frozen, and if in doing so the shelf life of the yeast is prolonged. The most convincing argument against freezing is that under normal conditions, there are temperature fluctuations in freezer units caused both by repeated opening and closing of the freezer door and, in contemporary freezer models, by the self-defrosting (freeze and thaw) cycle. These temperature fluctuations can cause damage to the yeast cell structure.

One topic upon which there is agreement is that if active dry or instant active dry yeast has been refrigerated, and is going to be rehydrated in lukewarm water, it is best to allow the portion of yeast to be used to come to room temperature prior to blending it with the lukewarm water. Otherwise, temperature shock might damage the yeast cells.

Unlike compressed yeast, which disperses in cold water without any problems, the temperature of the water during rehydration is important when working with dry yeast. When yeast is dried, the cell membrane becomes more porous. During rehydration, the membrane recovers. However, in the process of rehydration, some cell constituents are dissolved in the water used. The optimum water temperature for cell membrane restoration is 104 F. Warm water is effective in this process, because it leads to more rapid cell membrane recovery. Cold water impedes this process, because it slows membrane recovery and allows more cell constituents to leach out during the reconstitution process. The effect is not that great between 70 and 100 F, but at lower temperatures, approximately one-quarter to one-half of soluble yeast cell constituents can be lost. This leaching action effects yeast activity in the following manner: Most yeast enzymes remain, but the soluble chemicals are depleted, and it is these chemicals that promote enzyme activity. Furthermore, even under optimal conditions, glutathione is released or leached out of the cell and can effect dough consistency. Glutathoine contributes to dough slackening and can cause dough to soften and become sticky.

Guidelines for working with compressed yeast and active dry yeast appear relatively standardized for both commercial and home settings. This is not necessarily the case for instant active dry yeast as marketed to the home baker. The fineness and porosity of instant active dry yeast particles allow it to be dissolved quickly and homogenously into the dough for most applications. Lallemand is clear in its description of how to work with instant active dry yeast and the methods are easily adapted to home baking. The following is taken from the Lallemand Baking Update entitled "Instant Yeast":

"Rehydration

The recommended methods for rehydrating instant dry yeast are aimed at avoiding direct contact with excessive amounts of cold water. Using either warm water or a slow rehydration helps to optimize baking performance.

Blend with flour. The simplest method for using instant yeast is by blending the instant yeast thoroughly with the flour before adding (cold) water. The flour absorbs much of the water so that it doesn't come into direct contact with the yeast. Using this technique, the mixer can be started immediately after adding the water, and the dough can be checked at the end of mixing for undissolved yeast particles by stretching it into a thin film. When using ice-cold water, it is best to leave the yeast in the flour for about thirty minutes before adding the cold water and starting the mixer. This improves activity by giving the yeast time to absorb some of the moisture from the flour, as long as the temperature of the flour itself is not extremely cold.

Sprinkle on dough. Another method is to mix all the ingredients except the yeast for one to two minutes, then sprinkle the instant yeast on top of the partially mixed dough and continue mixing. Because the instant yeast is added at a time when most of the (cold) water has been absorbed by the flour, a cold shock is prevented. In this method, the exact timing for sprinkling the instant yeast is critical, and it is important to check if all the yeast particles have disappeared at the end of mixing. When using ice-cold water, it is best to wait three to five minutes after the yeast addition before restarting the mixer. This improves activity by giving the yeast time to slowly absorb moisture from the partially mixed dough.

Add to warm water. The traditional method for using conventional active dry yeast (ADY) can also be used with instant yeast. Blend one part instant yeast with three to four parts lukewarm (95-105F/35-40 C) water. Wait ten minutes, then stir and add the fully rehydrated instant yeast to the mixer. This method is useful with high-speed mixers where the very short mix time of five minutes or less does not allow for complete rehydration. It is also useful …to avoid direct contact with ice-cold water. Although the traditional method requires more time and attention, it gives the highest level of yeast activity…"

Fleischmann suggests that it takes slightly longer to rehydrate instant active dry yeast at the same dough temperature as compressed yeast or active dry yeast, since instant active dry yeast is the driest of bakers yeasts. In this case, a factor of an extra 4 degrees is suggested. For example, if a particular dough calls for a dough temperature after mixing of 78-80 F, then 82-84 F is recommended, i.e. (78 + 4 =82 and  80 + 4 = 84).  Alternatively, the same dough temperature may be maintained after mixing, but the dough should be allowed to ferment for a slightly longer period of time.

Although warm rehydration maximizes the performance of instant active dry yeast, companies such as Fleischmann and Red Star suggest that home bakers use water ranging in temperature from 120 to 130, which is excessive. Since, leaching of cell constituents is minimized during rehydration when water is between 70-100 F, using lukewarm to warm water temperature in the dough is advised. 

We have communicated with Fleischmann and have been informed that the vast majority of home baking complaints that Fleischmann receives about yeast failures stem from the dough being either too cold, or held at cold proofing temperatures.  While 120° F.  is certainly excessive for the experienced baker who has control of ingredients, weights, time and temperature, using this temperature does help the inexperienced baker to achieve a faster proof and and to obtain something tangible at the end of the baking process.  It is important to note that Fleischmann's recommendations for their experienced retail and commercial customers are dramatically different, and comport with The Artisan's findings.

Yeast Conversion
The yeast conversion ratio is 100 percent compressed yeast to 40 percent active dry yeast to 33% instant active dry yeast. When converting compressed yeast to active dry yeast or instant active dry yeast in a commercial setting, it is important to take the difference in dry matters into account by making up the difference in weight with water. Table 1 illustrates the conversion from compressed yeast to active dry yeast. (1 oz is rounded to 30 g in  the table)

Table 1

Compressed Yeast Active Dry Yeast Additional Water
     
3 oz (90g) 1 .20 oz (36 g)  1.80 oz (54 g)
6 oz (180 g) 2.40 oz (72g) 3.60 oz (108 g)
9 oz (270 g)  3.60 oz (108 g)     5.40 oz (162 g)
12 oz (360 g)  4.80 oz (144 g) 7.20 oz (216 g)
1 lb. (16 oz) (480 g) 6.40 oz (192 g) 9.60 oz (288 g)
1 lb. 8 oz (720 g)  9.60 oz (288 g) 14.4 oz (432 g)

Table 2 illustrates the conversion from compressed yeast to instant active dry yeast. (1 oz is rounded to 30 g in the table .)

Table 2

Compressed Yeast Instant Active Dry Yeast Additional Water
     
3 oz (90g) 1 oz (30 g) 2 oz (60 g)
6 oz (180 g) 2 oz (60 g) 4 oz (120 g)
9 oz (270 g)  3 oz (90 g)   3 6 oz (180 g)
12 oz (360 g)  4 oz (120 g) 8 oz (240 g)
1 lb. (16 oz) (480 g) 5.28 oz (158 g) 10.72 oz (322 g)
1 lb. 8 oz (720 g) 7.92 oz (238 g) 16.08 oz (482 g)

The companies specializing in yeast packaged for home baking recommend substituting 1 cube compressed yeast (0.6 oz) for 2 1/4 teaspoons active dry yeast for 2 1/4 teaspoons instant active dry yeast. As stated above, a more precise ratio is 100 percent compressed yeast to 40 percent active dry yeast to 33% instant active dry yeast. Table 3 provides a guide to converting compressed yeast to active dry yeast to instant active dry yeast. (We refer those interested in exact conversion measurements in ounces and grams to the Yeast Conversion Chart. The chart allows home bakers to choose whether or not to include additional water in the conversion.)  The ratio of active dry yeast to instant active dry yeast is 1.25:1.

Table 3

Compressed Yeast Active Dry Yeast Instant  Active  Dry Yeast
     
1 cube 2 1/2 tsp   2 tsp
3/4 cube 1 7/8 tsp   1 1/2 tsp
1/2 cube 1 1/4 tsp     1 tsp
1/4 cube  5/8 tsp  1/2 tsp

Functions of Yeast in the Breadmaking Process

There are three main functions of yeast in dough. They are leavening, dough maturation and development, and flavor development. These are achieved through the following steps (excerpted from Technology of Breadmaking edited by Stanley P. Cauvain and Linda S. Young).

All of the processes that have evolved for the manufacture of bread have a single, common aim. That aim is namely to convert wheat flour into an aerated and palatable food. In achieving this there are a number of largely common events that occur. Those specific to yeast are as follow:

Additionally leavening effects the volume, crust, texture, taste, wholesomeness and shelf life of leavened breads.

Dough Fermentation & Temperature
There is a specific relationship between yeast and temperatures. Like most living organisms, the metabolic activity of yeast ceases above and below certain fairly well defined temperatures. Its ability to leaven is also effected over a broad thermal range. These are summarized in Table 4 below.

Table 4

Temperature Activity
-20 C. (-4 F) Loss of Fermentation Capacity
< 20 C (68 F > 40 C (104 F) Growth Rate Significantly Reduced
20 C (68 F) - 27 C (81 F) Most Favorable Range For Yeast to Multiply
26 C (79 F) Optimum multiplication of Yeast Achieved
27 C (81 F) - 38 C (100 F) Optimum Fermentation Range
35 C (95 F) Optimum Fermentation Temperature
> 60 C (140 F) Yeast cells Die

The temperature of the fermenting medium also exerts a significant effect on the yeast's gassing rate. As a general observation, the fermentation rate increases with a rise in temperature up to a maximum of perhaps 100 to 105 F (38 to 41 C). The rates of carbon dioxide generation of dough without added sugar over a 3-hr period at various temperatures within the range of 81.5 to 95°F (28 to 35°C) are shown in Graph 1.  (Pyler)

Rapid assimilation by yeast of the free sugars available in the flour accounts for the initial  rapid activity shown in the first minutes. The subsequent drop-off in the rate that follows marks the exhaustion of the supply of this free sugar, and the period of adaptation of the yeast to the fermentation of the maltose that is being produced by the action of amylases on the damaged starch in the flour. After this period, the gassing rate rises again for about 3 hr, or when the maltose supply is also  depleted.  Interestingly , the fermentation is faster at 35 C (95 F) and  an earlier exhaustion of the maltose supply occurs.  Thus the final decline in the carbon dioxide is most pronounced here rather than at the bit higher temperature of 90.5 F. (32.5 C).

Once the dough or ferment temperature exceeds about 105°F (41 C),  the fermentation rate declines. Yeast enzymes are generally inhibited at this temperature.  Table 5 below (derived from Pyler) correlates the effects gas production and temperature in liquid ferments. As can be seen, gas production increases as the temperature rises to 100 F (38 C).  It then declines at higher temperatures. (Note: All temperatures are rounded)

Table 5

Temperature

Deg. C        Deg. F

 Maximum Gas Production Rate: In Millimoles CO /hr/g/Dry Yeast Solids Time to Maximum gas Production rate (in Min.)
29 84 20 150
31 88 23 135
33 91 24.5 135
36 96 25 120
38 100 26 90
40 104 22.5 75
42 108 20 30

Dough Development

Dough development is a relatively undefined term.  Among other things it addresses a number of complex changes in bread ingredients that are set in motion when the ingredients first become mixed. The changes are associated with first the formation of gluten, which requires both the hydration of the proteins in the flour and applied energy. The role of energy in the formation of gluten is not always fully appreciated. It is often erroneously associated with particular breadmaking processes, especially those which employ higher speed mixers.

Initially, gluten is formed when flour and water are mixed together. The proteins in the flour, glutenin and gliadin cross link, using the water as a vehicle to form gluten. Enhancing this gluten structure is important relative to developing a gas retaining structure in the bread. (Corriher) Energy is provided through the processes of fermentation, respiration and kneading. Simply put, gluten does not form spontaneously in that energy must be provided for its formation. There is no spontaneous combustion…at least not in breadmaking.

Cell Creation and Control Thereof in the Dough
The production of a defined cellular structure in the baked bread depends entirely on the creation and retention of gas bubbles in the dough. After mixing has been completed, the only 'new' gas which becomes available is the carbon dioxide gas generated by the yeast fermentation. Carbon dioxide gas has many special properties. At this point we are concerned with two: its high solubility and its relative inability to form gas bubbles. As the yeast produces carbon dioxide gas, the latter goes into solution in the aqueous phase within the dough.

If the carbon dioxide does not form its own gas bubbles how then does expansion of the dough through gas retention occur? Two other gases are available in significant quantities within the dough as a result of mixing. These are oxygen and nitrogen, both of which are derived from any quantities of air trapped within the dough matrix as it forms. In the case of oxygen, its residence time in the dough is relatively short since it is quickly used up by the yeast cells within the dough Indeed so successful is yeast at scavenging oxygen that in some breadmaking processes no oxygen remains in the dough by the end of the mixing cycle. Thus, the bread fermentation process is referred to as an anaerobic, alcoholic fermentation brought about by fermenting agents present in the dough, The rapid loss of oxygen from mechanically developed doughs has been illustrated previously for a wide range of nitrogen to oxygen ratios

With the removal of oxygen from the dough, the only gas that remains entrapped is nitrogen. Nitrogen plays a major role by providing bubble nuclei into which the carbon dioxide gas can diffuse as the latter comes out of solution. The number and sizes of gas bubbles available in the dough at the end of mixing will be strongly influenced by the mechanism of dough formation and the mixing conditions in a particular machine. The effects of mixer design are very important, but this is not within the scope of this presentation. At this stage it is only necessary to register the significant role that mixing will play in the creation, and/or manipulation of dough bubble structures.

Osmotic Pressure

The osmotic properties of a yeast cell are due to selective permeability of the cell wall with regard to solutions. This selectivity plays an important role in controlling the movement of nutrients into a cell. Nutrients are present in a medium in the form of ions, sugar, and amino acids. The permeability of the cell wall also permits the release of alcohol and carbon dioxide from the cell during fermentation.

High concentrations of sugars, inorganic salts, and other solubles inhibit yeast fermentation as a result of effects produced by high osmotic pressures. Basically, all fermentable sugars begin to exert an inhibiting effect on yeast when their concentration exceeds about 5% in the dough, with the degree of inhibition becoming progressively greater as the concentration of the sugar rises. This inhibitory effect is more pronounced with such sugars as sucrose, glucose and fructose than with maltose. The last sugar is a disaccharide that persists as such in the fermenting medium, and therefore exerts a lower osmotic pressure than the monosaccharides and the readily hydrolyzed sucrose, The sensitivity of yeast to osmotic pressure varies with different yeast strains, with some being better suited than others for fermenting sweet doughs with their high sugar contents.

Salt exerts a similar osmotic effect, except that some fermentation inhibition appears to set in at concentrations below the normal 2.0% level. A decrease in gas production occurred over a four (4) hour period  when the concentration of sodium chloride was increased from 1.5 to 2.5% in a straight dough. One percent (1%)  salt, based on flour, exerts an osmotic effect that is equivalent to that of 6% glucose.

Salt in concentrations over 1.5% exerts an inhibitory effect on yeast activity, either by its osmotic pressure or by a specific chemical effect. For this reason, Salt is generally withheld from the sponge in the sponge-and-dough process. Interestingly, it has been shown that at lower levels, rather than being detrimental, salt actually exerts a favorable influence on yeast fermentation, A series of studies have shown that the use of 0.5 to 1.0% salt in the sponge of a sponge-and-dough process resulted in reductions in the fermentation time, while at the same time producing a better quality bread than was obtained with a sponge containing 0.15% or no salt.

The Fermentation Process
Please note that the majority of scientific data available to us regarding sponge and dough fermentation is found in Baking Science and Technology by E. J. Pyler.  He describes sponge and dough as follows: "In the sponge-and-dough method, the major fermentative action takes place in a preferment, called the sponge, in which normally 50 to 70% of the total dough flour is subjected to the physical, chemical, and biological actions of fermenting yeast. The sponge is subsequently combined with the rest of the dough ingredients to receive its final physical development during the dough mixing or remix stage…. Sponge consistency may vary from stiff to soft or slack, depending on the baker's over-all expectations regarding its influence on final product quality" .

As bakers know, before any dough can yield a light, aerated loaf of bread, it must be fermented for a sufficiently long time to permit the yeast to convert the assimilable carbohydrates  into alcohol and carbon dioxide as the principal end products.

The most apparent physical change marking the course of fermentation in a dough is the steady increase in the volume of the dough mass. The sponge expands to four to five times its original volume before it recedes, assuming at the same time a light, spongy character. The findings described in Pyler relative to the gassing power of yeast action on carbohydrates are interesting and of value to the baker. For example, if 100 lb of flour will yield approximately 180 lb of dough, the degree of expansion in the dough during fermentation and proofing can be sustained by about 3.5% of fermentable carbohydrates, based on flour. Part of these carbohydrates may be comprised of the native sugars of flour, part may result from alpha-amylase action on damaged starch, and part may comprise added  sugar. Any sugar over and beyond the 3.5% level will show up as residual sugar in the finished bread.

In bread baking, fermentation occurs due to a conversion of sugars (technically, glucides or sugars, naturally present in the flour) to alcohol and carbon dioxide under the effect of commercial or naturally occurring yeast and bacteria. This is categorized as alcoholic fermentation. Figure 3  below outlines some of the basic chemical reactions which occur during fermentation. Not included in this schema is the conversion of sucrose to glucose and levulose by the enzyme invertase. Glucose and levulose are then subsequently converted to carbon dioxide and ethanol by zymase as shown  below..

Sugar Transformations (Rosada)
Simple sugars: The main simple sugars, glucose and fructose, represent about 0.5% of the flour. Yeast can directly assimilate them by penetration of the cell membrane. Simple sugars are transformed into alcohol and carbon dioxide by zymase, an enzyme naturally present in yeast cells. Because of this easy absorption, these sugars are the first ones used in the fermentation process. Their consumption takes place during the first 30 minutes or so at the beginning of the fermentation process.

Complex sugars: The two main types naturally present in flour, saccharose and maltose, represent approximately 1% of the flour. Because of their complex composition, these sugars will be used later on in the fermentation process. The lapse of approximately 30 minutes at the beginning of the fermentation period is necessary to achieve their enzymatic transformation into simple sugars. The enzymes involved are saccharase, which transforms saccharose into glucose and fructose, and maltase, which transforms maltose into glucose.

Very Complex sugars: The main very complex sugar is starch, which represents about 70% of the flour content. Two types of starch are found in flour: amylose and amylopectin. Amylose is degraded by the enzyme beta amylase into maltose, and in turn the maltose will be degraded into glucose by the maltase enzyme. Amylopectin is degraded by the alpha amylase enzyme into dextrin, after which the dextrin is degraded by the beta amylase into maltose. This maltose will them be degraded by the maltase into glucose.

The simple sugar, glucose, obtained during these transformations is used by the yeast to generate carbon dioxide and alcohol. During the fermentation process, most of the starches used are the ones damaged during the milling process. Because the particles are damaged, they can easily absorb water during the dough making process. This water contact triggers the enzymatic activity. A non-damaged particle of starch will only retain water at its periphery and not inside the particle itself.
 

Link to "Yeast - A Treatise" Section II

Last updated on02/03/2002